This application claims the priority of German patent document 197 03 629.5, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for autonomous on-board determination of the position of an earth orbitting satellite.
In prior art systems of this type, the determination and holding of the position of a satellite normally does not take place automatically and autonomously. That is, an earth station determines the deviation of the actual orbit from the desired orbit, and corresponding correction maneuvers are then manually commanded from earth.
To save cost, it may be advantageous to carry out the tasks of "orbit determination" and "orbit correction" autonomously on board; that is, without the intervention of an earth station. The results of investigations of different methods for the board-side determination of orbit parameters and their correction largely independently of interventions of the earth station are described in P. Maute et al., "Autonomous Geostationary Station Keeping System Optimization and Validation", IAF-88-327,39. Congress of the International Astronautical Federation, 1988, Bangalore, India and J. Potti et al., "On-Board Autonomous Station Keeping (OBASK) Executive Summary, ESTEC Contract No. 9675/91/NL/JG GMVSA 2064/93, GMV S.A. 1993. However, in addition to measuring equipment which is already available on board such craft for the purpose of controlling the attitude (particularly earth and sun sensors), most systems of this type require additional measured-value generators which are used solely, or at least predominantly, for the purpose of determining orbit parameters. Particularly, the use of additional star sensors was found advantageous for this purpose.
One exception is indicated in K.D. Mease, et al., "An Approach to Autonomous On-Board Orbit Determination", The Journal of the Astronautical Sciences, Vol. 33, No. 2, pp 163-178, 1985, which indicates that the satellite orbit can be determined in a complete manner by means of a biaxially measuring earth sensor, a two-axis sun sensor and a Kalman filter. The sun sensor is mounted on the drive of the solar generator so that, in the case of the known position of the drive, the sensor measurements can be transformed into the system of body coordinates.
The known methods and systems for the autonomous orbit determination suffer from the following deficiencies:
The installation of additional measuring instruments, particularly star sensors, results in additional costs, weight and complexity of the measuring system. PA1 In the case of a polaris sensor, system-technical difficulties occur because, as the result of sun reflections, light scatter shields must be mounted on the solar generators which also result in additional weight. PA1 The use of a sun sensor mounted on the drive of the solar generator requires that the angular position of the driving motor with respect to the satellite be known precisely in order to transform the measuring information into the body-fixed satellite system. However, in the case of conventional driving motors (stepper motors), this generally cannot be ensured with sufficient precision. PA1 It can be demonstrated that the complete orbit information cannot be determined by means of a respective biaxial earth and sun sensor measurement. PA1 1. The system consists of a biaxially measuring earth sensor which defines the z-axis (yaw axis) of the satellite and of several biaxially measuring sun sensor measuring heads which are arranged on the satellite structure such that, with the exception of earth shadow phases, they supply a direction vector measurement. PA1 2. The out-of-plane movement of the satellite is propagated by means of a precise on-board model of the satellite dynamics including natural disturbance forces and thrusts during maneuvers. PA1 3. In order to compensate sensor uncertainties and influences of thermal deformations, the satellite orbit is precisely measured at regular intervals (approximately 3 months). By means of this information, a calibration function (time function for a day) is determined by means of which the measured values are compensated. PA1 4. The in-plane movement is not only propagated but the propagation is supported by measured data (estimation), and the effects of out-of-plane movement and thermal deformations are taken into account, by means of the data of the calibration function (k(t) or of the propagated out-of-plane movement. PA1 5. It is possible for the earth station to switch off the automatic and autonomous orbit determination and position holding as soon as larger deviations occur during the autonomous operating phase between the position computed autonomously on-board and the position (passively) monitored by the earth station.
It is therefore an object of the present invention to determine the orbit (position) of a satellite as far as possible autonomously on board the spacecraft, by using normally existing sensors (biaxially measuring earth and sun sensors), in order to facilitate optimal correction maneuvers by means of this orbit information.
In the following, it is assumed that the earth and sun sensors supply direction vectors of length I in the direction of the center of the earth and in the direction of the sun, respectively. Sensor errors, installation errors of the sensors, and relative position errors of the sensors because of thermal deformations of the satellite structure interfere with the directional measurements. The object of the invention therefore also includes the provision of methods for identifying and eliminating such error influences.
These and other objects and advantages are achieved by the process and apparatus according to the invention, which is characterized by the following features:
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.